1. Field of the Invention
This invention relates generally to a brush assembly including a brush wear detector for use on a dynamoelectric machine and more particularly to a magnetic type brush wear detector which energizes an indicating signalling circuit.
2. Description of the Prior Art
Rotating electrical machinery, and more particularly direct current motors, include a stator and a rotating armature having a commutator which is electrically coupled to an external electric circuit through one or more brushes which contact the commutator. As the commutator rotates, the contact surfaces of the brushes wear down to the point where replacement is required to prevent damage to the commutator and to ensure efficient power transfer.
Brush wear detectors are well known and generally comprise various types of mechanical and electrical circuit arrangements which act to signal the fact that the brush has been worn away to a length at which replacement is required. One known typical example comprises a brush wear detector having a self winding biasing spring mounted on the brush holder which operates to urge the brush inwardly against a rotating slip ring or commutator of an electrical machine. The winding action of the spring is used as an actuator for electrical circuit means which operates to energize a visual type of indicator. An example of such apparatus is shown and described in U.S. Pat. No. 4,488,078, entitled, "Brush Wear Detector", which issued to Ronald C. Orton, on Dec. 11, 1984, and which is also assigned to the present assignee.
Another example of brush wear detector includes a read switch mounted at the bottom of the brush holder and a pair of cylindrical magnets disposed within holes formed (e.g., drilled) in the brush itself. When the brush wears sufficiently, the magnets approach the reed switch, activating that switch to effect a signal indicating a worn brush condition. This arrangement requires modification to standard brush which can present difficulties during field replacement and is not practical in those situations where a large number (e.g., up to ten) of individual brush elements are included in the same brush box or holder.
It is an object of the present invention, therefore, to provide an improvement in brush wear detectors for use with dynamoelectric machines.
It is another object of the invention to provide a brush wear detector for use with brush holder assemblies of electrical machinery such as direct current motors.
A further object of the invention is to provide a brush wear detector which is inexpensive to fabricate but nevertheless operates in a reliable and efficient manner.
And still another object of the invention is to provide a brush wear detector which overcomes certain disadvantages inherent in known prior art apparatus for detecting brush wear.
An additional object is to provide a brush assembly including a brush wear detector which readily permits replacement in the field using, basically, standard brush elements.